The invitation is meant to build bridges between the royal family and some of the most deprived parts of British society. Twelve-hundred handpicked members of the public – including inner-city youth workers from Coventry and community leaders from Bolton – will crowd inside the grounds of Windsor Castle to be among the first to see Prince Harry and Meghan Markle emerge as a married couple on Saturday 19 May.

But it seems the hospitality won’t extend far. Representatives of the royal family, which has an estimated net worth well in excess of £400m, have sent letters to these guests encouraging them to bring their own picnics, in a move described by one invitee as “unfathomable”.

Debrett’s guide to etiquette advises at least six canapés per person pre-lunch at a wedding, but guests have been encouraged in letters from lord lieutenants, the Queen’s representatives in the counties, “to bring a picnic lunch as it will not be possible to buy food and drink on site”.

The crowds are expected to spend at least four and a quarter hours on the verges outside St George’s chapel, filling a space inside the castle grounds that would otherwise show up on TV coverage as empty.

Saeed Atcha, 21, the founder of Xplode, a Bolton-based youth magazine set up after the 2011 riots, said some of the disadvantaged people his charity helped were bemused by the decision.

“They were saying how come they have this money and you have to bring a picnic,” he said. “I am of the same opinion. It’s unfathomable.”

He said he had been checking on internet maps and saw there was a supermarket and a fast-food outlet nearby. “There’s a McDonald’s, but I’m not sure I’ll be able to bring in a filet meal. Maybe there will be a U-turn.”

Kensington Palace indicated this week that some refreshments and snacks would be available, but declined to elaborate.

It matters not to Rashid Bhayt, 38, the leader of a youth charity from Coventry who will attend the wedding with his wife, because he will be observing Ramadan, the dawn-to-dusk fast observed by Muslims. “If you hear I fainted, it won’t be because I was starstruck,” he said.

Last month Kensington Palace announced that Meghan and Harry wanted to shape their wedding day “to allow members of the public to feel part of the celebrations too”. The invited crowd inside the castle perimeter will include people “from a broad range of backgrounds and ages, including young people who have shown strong leadership and those who have served their communities”.

They will play a starring role in TV pictures beamed around the world to an audience expected to be in the hundreds of millions – although throwing rice or confetti is not encouraged.

The crowd will be bolstered by 1,140 palace staff and people who live and work in and around Windsor Castle, 100 schoolchildren, and 200 people associated with charities in which the couple are involved.

At 2pm, after the wedding, the 600 guests inside the chapel will go to a lunchtime reception laid on by the Queen in the castle, while the crowd outside will go home or mill about in Windsor.

Paucity on the catering front has not dimmed invitees’ enthusiasm for being part of a crowd that Harry and Meghan are keen should reflect Britain’s diversity. The 200 guests associated with the couple’s charities will have the prime spots closest to the chapel doors. They will include representatives of Invictus, the sports charity for injured servicemen and women, Sentebale, a humanitarian charity in Lesotho, and Wellchild, a charity for seriously ill children.

Atcha said he set up his magazine charity when he was 15 after young people were branded “feral animals” and “scum” in some of the the media coverage of the 2011 riots. The charity has given 3,000 people opportunities to work on the quarterly magazine. Before Meghan and Harry got engaged he had seen them speak separately, her about racism and him about youth activism.

“When I saw they were together I thought wow, they are a power couple,” he said. “There’s lots to be done to help the next generation support the royal family. When the Queen is no longer here, I am not sure young people will connect with Charles the way they do with Harry and Meghan, who just seem like a normal couple.”

He said the young people he worked with “don’t really know what Prince Charles does, but are excited about the royal wedding”. He said some resented the royals’ privilege.

“They have the view that they are disadvantaged and ask what have they ever done for me,” he said. “They see how much they are spending to renovate a palace and they are living in a council house with all of their siblings and have never been on holiday or even left Greater Manchester. I suggest they research the programmes the royal family have and see if you can get involved.”

Bhayt said he worked with young people in Coventry who were “alienated from society”, often affected by cuts to youth centres and sports programmes that left them with fewer activities to occupy them and more susceptible to antisocial behaviour and crime.

“In Coventry and beyond this invitation has made people feel connected to the wedding,” he said. “That is important for the royals at the moment. They are really reaching out.”

He said the initiative would help in “building a bridge between higher society and us at the grassroots”.